I think the extra fiber is good for you and can help you be more regular. The problem I've had on a reboot is because we consume so little fiber, our digestive system someones holds it in the large intestine until there is enough to warrant a BM. We can feel constipated and have little BMs every couple of days. I personally prefer a higher fiber intake so my digestive system is always running, even if at a slower speed. That can help with the weight loss, make it easier to have the salad or soup from time to time, and makes it easier to transition out of the reboot.
Our digestive systems will turn up and down just enough to handle whatever we consume. Extra fiber causes it to rev up just enough to handle the fiber. A small bowl of soup or salad causes it to turn on just enough to handle the input, and then it settles down again.
It's the big changes that will cause a problem. If we have a large meal in the middle of a reboot, our digestive system needs to crank up to full-speed-ahead to handle it, and it will probably take a couple of days to fully process it and settle down again. That can wreak havoc on a reboot and cause brutal hunger for a day or two.
The rule of thumb (my rule of thumb at any rate) is that whatever we eat/drink:
1) Don't make big changes to how much we consume from day to day.
2) Whatever/how much you eat/drink, try to do it consistently every day.
If you take extra fiber, do it every day consistently. Same for the soup or salad. Our tummys like consistency and will get upset if we break the routine.